[MetricSystem] fix typo: missing enum in "from_value" function

tomoscan/unitsystem/metricsystem.py

@@ -98,7 +98,7 @@ class MetricSystem(_Enum):
         if isinstance(value, str):
             return cls.from_str(value=value)
         else:
-            _Enum.from_value(value=value)
+            return _Enum.from_value(value=value)
 
     @classmethod
     def from_str(cls, value: str):